PureOS isn’t locked and won’t prevent you from running any software by any technical means. It’s made without non-free and its repos do not come with non-free software, sure, but that’s not a measure of locking or prevention.
I’m not sure what snapd has to do with this whole thing either, that’s another package management technology which is mostly included with Ubuntu. I’ve never heard of anyone willingly installing it on Debian 
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